This invention relates generally to high power transistor amplifiers and more particularly to packaging arrangements for removing heat from transistors used in such amplifiers.
As is known in the art, high power microwave amplifiers have a wide range of applications. Many applications, such as Continuous Wave (CW), (i.e., high duty cycle), Class C operation, require that, for practical use such amplifiers must operate with high power, high efficiency and be of low cost. These applications require a technique for efficiently removing heat from the transistor. One technique used to remove such heat is by the packaging arrangement shown in FIGS. 1A and 1B. As shown therein, a semiconductor chip 10, having a bipolar transistor configured in a common base configuration, is shown. The semiconductor chip 10 is disposed on a thermally conductive, electrically insulating, beryllium oxide (BeO), heat sink 12. The heat sink 12 is mounted to a thermally conductive mounting flange 14. A dielectric corral 16, here a ceramic such as alumina, has an aperture 18 in an inner region thereof. The chip 10 with the heat sink 12 is disposed in the aperture 18 so the outer portion of the coral 16 encloses or encases, the sidewalls of the beryllium oxide heat sink 12. The corral 16 has electrical conductors 21 passing therethrough to enable connection of input signals, and bias voltages to the amplifier of chip 10 and to enable the amplified signal to be coupled out of the packaging arrangement. More particularly, the amplifier of the chip 10 is electrically connected to the conductors 21 by wires 22 (FIG. 1B). A cover 24, for example Kovar or Alloy 42 material, is used to shield the amplifier of chip 19. The width of the aperture 18 is typically less than .lambda./4 where .lambda. is the wavelength of the nominal microwave operating frequency of the amplifier.
Another packaging arrangement is shown in FIG. 2. Here, a diamond insert 26 is placed in the central region of the beryllium oxide heat sink 12. The semiconductor chip 12 is mounted to the upper surface of the diamond insert 26 and the lower surface of the diamond insert 26 is mounted on the mounting flange 14. Thus, the diamond insert 26 is enclosed, or encased around the sidewalls thereof by the beryllium oxide corral 16. Testing such an arrangement showed a temperature reduction of less than 3 degrees Centigrade when operating under the same conditions as the packing arrangement shown in FIGS. 1A and 1B.
In spite of this improvement, such device has not been found adequate for Class C operation with high duty cycle, (i.e., substantially CW) operation, at high power levels. Further, beryllium oxide is a highly toxic, and therefore undesirable, material.